Viscometer

ABSTRACT

A viscometer for measuring liquid viscosities based upon rotational deflections of a suspended bob. The viscometer comprises a deflection indicator, a deflection reader located at a spaced relative position with respect to the indicator, a rotating element which rotates in unison with the bob and includes either the deflection indicator or the deflection reader, and a magnetic bearing assembly which prevents any substantial change in the spaced, relative position of the deflection reader with respect to the deflection indicator.

[0001] This application claims the benefit of prior filed, copendingU.S. provisional patent application Ser. No. 60/200,919, filed May 1,2000.

FIELD OF THE INVENTION

[0002] The present invention relates to viscometers of the type formeasuring liquid viscosities based upon rotational deflections of asuspended bob.

BACKGROUND OF THE INVENTION

[0003] There have been many prior designs of viscometers that utilizethe torsion wire principle. Early designs made use of a relatively heavybob suspended on a thin wire. The bob was suspended in the liquid to bemeasured and a container holding the liquid was rotated. The twistingforce imparted to the bob via the liquid was measured by a deviceattached to the wire. In these designs, the weight of the bob was reliedon to keep the wire centered. This, in turn, limits their use to ‘thinliquids’ and thin wires.

[0004] Other designs have used thicker wires or rods but these designslack the sensitivity required for the fluids they desire to study.Designs such as U.S. Pat. No. 4,299,118 employed a bob mounted on a rodwith the rod attached to a fine torsion spring. The rod is centered byball bearings which are also used to center a rotatable sleeve,concentric with and centered about, the bob. This arrangement allows anyfluid container to be used but inaccuracies occur because of mechanicalfriction transmitted from the driven sleeve to the bob through thebearings.

[0005] More recently, designs have appeared such as my earlier U.S. Pat.No. 5,763,766 where multiple fixation points are used to tension andcenter thicker wires. However, because the drive to the rotatingcontainer is from beneath the bob, special containers have to be used tocontrol the temperature of the sample. The design was modified in U.S.Pat. No. 6,070,457 to replace the lower mechanical fixing point for thewire with a concentric repelling magnet arrangement and, in fact, bearsa striking similarity to the apparatus described in U.S. Pat. No.4,045,999, In both of these examples, special containers are stillrequired for controlling the sample temperature.

[0006] The entire disclosures of U.S. Pat. Nos. 4,045,999, 5,763,766,and 6,070,457 are incorporated herein by reference.

[0007] It will be deduced by those skilled in the art that, althoughangular torsion or deflection is measured by an electronic non-contactsensor and transducer, the path traveled by the sensor is, in fact, anarc and therefore the response of the transducer cannot be linear due tothe curvature of the arc. As noted by Simon and Heflinger, stablelevitation (or repulsion) of one magnet by another is prohibited byEarnshaw's Theorem and this effect is seen as a tilt of tube 66 in U.S.Pat. No. 6,070,457. The combination of these two effects means that theinstrument described in U.S. Pat. No. 6,070,457 requires a sophisticatedcalibration routing to linearize the output from the transducer and, asa consequence, the instrument must be manually ‘zeroed’ before eachsample measurement.

[0008] Prior viscometers of the types discussed above are illustrated inU.S. Pat. Nos. 1,192,861; 1,236,706; 1,281,042; 2,203,132; 2,303,162;2,398,574; 2,957,339; 3,435,666; 4,045,999; 4,242,086; 4,299,118;5,763,766; and 6,070,457 and in European Application Nos. 007427;311301; 384792; and 449586 and PCT Application Nos. WO 91/06364; WO91/14168; WO 92/10763; and WO 92/06365.

[0009] It is clear from the teachings of these patents that the authorsconsider the centralizing of the bob to be of importance and none haveconsidered the location of the torque sensing device. In many of theabove examples mirrors or sensors are simply attached to the torsionwire and it is clear that any bowing of the wire or off center orbitingwill translate into a movement of the sensor that is not due to thefluid under investigation.

SUMMARY OF THE INVENTION

[0010] The apparatus of the present invention represents an improvementover the viscometer designs of the prior art and, in particular, thePalmer U.S. Pat. No. 4,045,999 and Robinson U.S. Pat. No. 6,070,457. Theapparatus of the present invention uses any container for the fluidunder test, rather than the specially designed temperature control unitand rotating cup of Robinson.

[0011] The apparatus of the present invention preferably uses a lineardistance measurement device which is an optical encoder system of highaccuracy. The use of this device eliminates calibration routines and anysubjective intervention by an operator. In one embodiment of theinvention, a freely swinging wire can be used. The encoder scale isattached to the end of the wire and a housing is attached to some pointon the wire. The encoder read head (preferably wireless) is mounted onthe housing opposite the scale. The two components are coupled togetherwith a magnetic bearing. By fixing the encoder scale and the read headin this manner, even if the wire swings, the two will move as one andthus record no relative movement between each other.

[0012] In a first aspect, the present invention provides a viscometerfor measuring liquid viscosities based upon rotational deflections of asuspended bob, wherein the improvement comprises: a deflectionindicator; a deflection reader located at a spaced relative positionwith respect to the deflection indicator effective for reading thedeflection indicator; and a rotating element mounted for rotation inunison with the bob. The rotating element includes one of the deflectionindicator and the deflection reader. The improvement further comprises amagnetic bearing assembly which retains at least the rotating element ina manner effective to allow the rotating element to rotate in unisonwith the bob while preventing any substantial change in the spacedrelative position of the deflection reader with respect to thedeflection indicator. As used herein and in the claims, the phrase “anysubstantial change” in the spaced relative position of the deflectionreader with respect to the deflection indicator refers to any change inrelative position exceeding the critical position of tolerances betweenthe indicator and the reader.

[0013] In a first embodiment of this aspect of the invention, theimprovement further comprises a suspended housing which holds the otherof the deflection indicator and the deflection reader. The rotatingelement is preferably positioned within the suspended housing.Additionally, the magnetic bearing assembly preferably comprises a firstmagnet included in the rotating element and a second magnet positionedin the suspended housing adjacent to and spaced apart from the firstmagnet. The magnetic bearing assembly more preferably comprises a thirdmagnet positioned in the suspended housing above and spaced apart fromthe first magnet with the second magnet being positioned below the firstmagnet. Further, the rotating member preferably does not contact thesuspended housing.

[0014] The deflection indicator employed in the inventive apparatus ispreferably a deflection scale and is most preferably an optical encoderscale. The deflection indicator is preferably included in the rotatingelement.

[0015] In a second embodiment of the first aspect of the invention, theimprovement further comprises a suspended, rigid structure having thebob extending from a lower end thereof with the rotating element beingretained on the rigid structure. This embodiment preferably furthercomprises a frame such that the magnetic bearing assembly comprises afirst magnet retained on the rigid structure and a second magnet held inthe frame at a position spaced above the first magnet. The magneticbearing assembly also preferably comprises a third magnet held in theframe at a position spaced below the first magnet. The rigid structureis preferably suspended through the second and third magnets such thatthe second and third magnets surround but do not contact the rigidstructure. The magnetic bearing assembly also preferably comprises afourth magnet retained on the rigid structure below the rotating elementwith the first magnet being retained on the rigid structure above therotating element and the third magnet being spaced below the fourthmagnet. The magnetic bearing assembly most preferably further comprisesa fifth magnet retained on the suspended, rigid structure at a positionabove and spaced apart from the second magnet.

[0016] The second embodiment can further comprise a flexible suspensionelement extending into the upper end of an interior passage provided inthe rigid structure. The flexible suspension element can be, forexample, a torsion wire. Alternatively, the second embodiment canfurther comprise at least one torsion spring connection between an outerportion of the rotating element and the frame.

[0017] In a second aspect, the present invention provides a viscometerfor measuring liquid viscosities based upon rotation deflections of asuspended bob wherein the improvement comprises: a frame for suspendingthe bob; a sleeve rotatably positioned around the bob; and a sleeveholder having a lower portion from which the sleeve extends and havingan upper portion rotatably retained in the frame such that the sleeveholder and the sleeve can be rotated by driving the sleeve holder at alocation above the bob. This improvement preferably comprises a pulleysecured on the sleeve holder above the sleeve for driving the sleeveholder. The improvement also preferably includes a bearing whichrotatably retains the upper portion of the sleeve holder in the framesuch that the pulley is positioned above the bearing.

[0018] Further objects, features, and advantages of the presentinvention will be apparent to those skilled in the art upon examiningthe accompanying drawings and upon reading the following description ofthe preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 provides a cutaway, elevational illustration of a firstembodiment of the inventive viscometer.

[0020]FIG. 2 provides a cutaway, elevational illustration of a secondembodiment of the inventive viscometer.

[0021]FIG. 3A provides a cutaway, elevational illustration of a thirdembodiment of the inventive viscometer.

[0022]FIG. 3B provides a view of the third embodiment as seen fromperspective 3B-3B shown in FIG. 3A.

[0023]FIG. 4A provides a cutaway, elevational illustration of a fourthembodiment of the inventive viscometer.

[0024]FIG. 4B provides a top view of the fourth embodiment as seen fromperspective 4B-4B shown in FIG. 4A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] One version of the apparatus is shown in FIG. 2. In thisarrangement, which is the simplest demonstration of the principle, atorsion spring wire 1 is suspended from a point 2 on the frame. Ahousing 3 is attached to the wire at point 4. There is sufficientclearance between points 2 and 4 to allow the wire to swing freely as apendulum. An encoder optical scale 50 is mounted on a pulley 5 and thepulley 5 attached to the end of the wire 5 at point 6. A bob 7 is alsomounted on the pulley 5 concentric with, and below, the wire 1 fixingpoint 6. The read head 8 for the encoder system is mounted on thehousing 3 in a position appropriate for reading the scale 50 on thepulley 5.

[0026] As so far described, it should be apparent that, if wire 1 swingsas a pendulum, then the bob 7 and pulley 5 can swing independently ofthe read head 8 and housing 3.

[0027] To enable the encoder system to function, critical positionaltolerances have to be maintained between the scale on the pulley 5 andthe read head 8. This is achieved in the present invention by using theattractive forces of, preferably, circular magnets of the same diameterposition in the housing 3 at 9 and 10, and in the pulley 5 at 11,concentrically about the wire 1. They are arranged so that magnet 10 isattracted to magnet 11, and magnet 11 is attracted to magnet 9. Thestrength of the attraction is varied by altering the distance betweenthe pairs of magnets and is adjusted so that, if the wire 1 swingsfreely, the relative positions of the scale 50 and read head 8 aremaintained because of the strength of attraction between the magnets 9,11 and 10.

[0028]FIG. 1 shows the apparatus in the preferred embodiment. In FIG. 1,a torsion spring wire 1 is attached to the frame at 2. A housing 3,which is essentially a threaded tube, is attached concentric with thewire to the end of the wire at 6. The opposite end of fixture 6 has aconcentric mounting for the bob 7. Using circular magnets of the samediameter, a magnet holder 14 and magnet 11 are screwed onto the housing3. The pulley 5 to which an optical encoder scale 50 is fixed to thecircumference is then screwed up to magnet 11. The assembly is held inplace by another holder 15 and magnet 12. Magnets 11 and 12 are arrangedso as to attract each other and the position on the housing 3 is suchthat the scale 50 is opposite the read head sensor 8. A further holder16 and magnet 13 are attached to the end of the housing above magnet 9and an additional magnet 25 is attached to housing 3 below magnet 10.

[0029] As described so far, it is clear that the open end of the housing3 is free to take up any position with respect to the wire 1 (limited bythe diameter of the tube). The encoder scale 50 and therefore pulley 5are stabilized and positioned concentrically with the wire (and therebyto the read head 8) by the use of two further similar magnets 9 and 10.These are positioned in the frame concentric with the wire mountposition 2. The whole is arranged such that magnet 10 is attracted tomagnet 11, magnet 11 is attracted to magnet 12, magnet 12 is attractedto magnet 9, and magnet 9 is attracted to magnet 13. The strength of theattraction (adjusted by varying the distance between magnets) is madesuch that there is no movement of pulley 5 toward or away from the readhead 8.

[0030] In this embodiment, it is possible to provide a rotating sleeve17 mounted concentrically with the bob 7 in a holder 18. The holder 18is mounted in the frame using a ball bearing 19. A timing belt pulley 20is mounted at the end of the holder and driven via a motor and timingbelt (not shown). This arrangement, in conjunction with a liftingplatform, allows the use of any suitable fluid container, it only beingnecessary for the fluid level to cover the bob 7.

[0031]FIG. 3 shows a variant of the apparatus in which the torsionspring wire 1 can be replaced with a cord with no spring properties atall, such as nylon or silk. In this embodiment, the restoring force isprovided by coil springs 23, 24 mounted to arms 21, 22 which, in turn,are mounted to pulley 5. The other ends of the springs 23, 24 areattached to the frame.

[0032]FIG. 4 shows the use of a passive magnetic bearing 54 described inU.S. Pat. No. 5,894,181 so that the torsion spring wire or cord can bedispensed with altogether. Restoring springs 56 can become extremelyfine allowing superb sensitivity.

[0033] Thus, the present invention is well adapted to carry out theobjects and attain the ends and advantages mentioned above as well asthose inherent therein. While presently preferred embodiments have beendescribed for purposes of this disclosure, numerous changes andmodifications will be apparent to those skilled in the art. Such changesand modifications are encompassed within the spirit of this invention asdefined by the appended claims.

What is claimed is:
 1. In a viscometer for measuring liquid viscositiesbased upon rotational deflections of a suspended bob, the improvementcomprising: a deflection indicator; a deflection reader located at aspaced relative position with respect to said deflection indicatoreffective for reading said deflection indicator; a rotating elementmounted for rotation in unison with said bob, said rotating elementincluding one of said deflection indicator and said deflection reader;and a magnetic bearing assembly which retains at least said rotatingelement in a manner effective to allow said rotating element to rotatein unison with said bob while preventing any substantial change in saidspaced relative position of said deflection reader with respect to saiddeflection indicator.
 2. The viscometer of claim 1 wherein theimprovement further comprises a suspended housing which holds the otherof said deflection indicator and said deflection reader.
 3. Theviscometer of claim 2 wherein the improvement further comprises therotating element being positioned within said suspended housing.
 4. Theviscometer of claim 2 wherein said magnetic bearing assembly comprises afirst magnet included in said rotating element and a second magnetpositioned in said suspended housing adjacent to and spaced apart fromsaid first magnet.
 5. The viscometer of claim 4 wherein said secondmagnet is positioned below said first magnet and said magnetic bearingassembly further comprises a third magnet positioned in said suspendedhousing above and spaced apart from said first magnet.
 6. The viscometerof claim 2 wherein said rotating member does not contact said suspendedhousing.
 7. The viscometer of claim 1 wherein said one of saiddeflection indicator and said deflection reader included in saidrotating element is said deflection indicator.
 8. The viscometer ofclaim 1 wherein said deflection indicator is a deflection scale.
 9. Theviscometer of claim 8 wherein said deflection scale is an opticalencoder scale.
 10. The viscometer of claim 1 wherein the improvementfurther comprises a suspended rigid structure having said bob extendingfrom a lower end thereof, said rotating element being retained on saidrigid structure.
 11. The viscometer of claim 10 wherein the improvementfurther comprises a flexible suspension element and said rigid structureincludes an interior passage having an upper end into which saidflexible suspension element extends.
 12. The viscometer of claim 11wherein said flexible suspension element is a torsion wire.
 13. Theviscometer of claim 10 further comprising a frame and wherein saidmagnetic bearing assembly comprises a first magnet retained on saidrigid structure, a second magnet held in said frame at a position spacedabove said first magnet, and a third magnet held in said frame at aposition spaced below said first magnet.
 14. The viscometer of claim 13wherein said rigid structure is suspended through said second and saidthird magnets such that said second and said third magnets surround butdo not contact said rigid structure.
 15. The viscometer of claim 13wherein: said first magnet is retained on said rigid structure abovesaid rotating element; said magnetic bearing assembly further comprisesa fourth magnet retained on said rigid structure below said rotatingelement; and said third magnet is spaced below said fourth magnet. 16.The viscometer of claim 15 wherein said magnetic bearing assemblyfurther comprises a fifth magnet retained on said suspended rigidstructure at a position above and spaced apart from said second magnet.17. The viscometer of claim 10 wherein the improvement further comprisesat least one torsion spring connected between an outer portion of saidrotating element and said frame.
 18. In a viscometer for measuringliquid viscosities based upon rotational deflections of a suspended bob,the improvement comprising: a frame for suspending said bob; a sleeverotatably positioned around said bob; and a sleeve holder having a lowerportion from which said sleeve extends and having an upper portionrotatably retained in said frame such that said sleeve holder and saidsleeve can be rotated by driving said sleeve holder at a location abovesaid bob.
 19. The viscometer of claim 18 wherein the improvement furthercomprises a pulley secured on said sleeve holder above said sleeve fordriving said sleeve holder.
 20. The viscometer of claim 19 wherein theimprovement further comprises a bearing which rotatably retains saidupper portion of said sleeve holder in said frame and wherein saidpulley is positioned above said bearing.